More particularly, the invention addresses the field of acquisition of a visual image of the tires intended to be fitted to rolling vehicles. These tires usually have a black color because of the use of carbon to reinforce the elastomer mixtures on which the production of said tires is based.
Also, it is particularly awkward to interpret the images acquired with the aid of sensors sensitive to the reflection of light on the surface of the tire. The light effects generated by the relief of the tire, by grease marks, the various marks, the differences in shades of black, or by localized discolorations, can be easily confused when the raw image supplied by a camera is analyzed undiscerningly.
Various methods of image acquisition have therefore been disclosed for the purpose of supplying data as pertinent as possible to a digital processing means capable of comparing this image with a reference image in order to determine the conformity of the tire to be analyzed or to find in this image anomalies that are present on the surface of the tire.
Publication EP 1 120 640 proposes using two separate cameras that are dedicated respectively to the acquisition of the data relating to the relief and to the acquisition of the data relating to the appearance, that is to say to the data such as the color, the gray level or the brightness. The surface of the tire is lit by two sources of light each assigned to a camera. To prevent interference between the light sources it is proposed to work with different light wavelengths or to circumferentially offset the cameras and their light source.
This solution, which requires relatively bulky means, also has the disadvantage of engaging considerable computing means in order to superpose the images originating from the two acquisition means.
Publication EP 1 477 765 tries to remedy this difficulty by proposing an inspection means comprising a single camera of the matrix type associated with a slot lighting in order to carry out a single acquisition and simultaneously determine the relief and the brightness of the surface to be inspected. The determination of the brightness is based on the observation of the fact that the differences in coloration on the surface of a tire such as the grease marks have the effect of increasing or reducing the dispersion of the wavelength of the reflected image.
This acquisition method nevertheless has the drawback of being limited to the brightness analysis only, and of proposing the use of a matrix camera of which the depth of field is difficult to adjust to the curvature of the tire, and of which the resolution is inferior to that of a linear camera. Moreover, being limited to a single light source, the analysis of the zones of great relief, such as the sculptures, leaves zones of shadows that are sources of inaccuracy and the choice of the angle of observation is reduced.
To solve this problem, publication WO/2005/050131 describes two alternative solutions. According to a first alternative, it is proposed to light the surface to be analyzed with the aid of a slot light oriented substantially perpendicularly to the surface and to have two cameras oriented in two directions forming opposed angles relative to the direction of the incident light. According to a second alternative, it is proposed to capture the reflected light with the aid of two mirrors placed in two opposite directions. The light originating from these two mirrors is then recombined with the aid of a set of prisms in the direction of a single camera.
However, these solutions do not make it possible to eliminate all of the shadow zones and require the recombining of the two images, either digitally when the images originate from the two cameras, or by a fine adjustment of the minor set. Moreover, as is mentioned above, the resolution of the matrix cameras remains weak and is combined with the loss of dynamics associated with the complexity of the optical chain.